A pointer to a variable that receives the size of the data that is stored in the output buffer, in bytes.

If lpOverlapped is NULL,
lpBytesReturned cannot be NULL. Even when an operation
returns no output data and lpOutBuffer is NULL,
DeviceIoControl uses
lpBytesReturned, which makes the value of lpBytesReturned
meaningless.

If lpOverlapped is not NULL,
lpBytesReturned can be NULL. If this parameter is not
NULL and the operation returns data, lpBytesReturned is
meaningless until the overlapped operation is complete. To retrieve the number of bytes returned, call
GetOverlappedResult.

If hDevice is associated with an I/O completion port, you can retrieve the number
of bytes returned by calling
GetQueuedCompletionStatus.

If hDevice is opened without specifying
FILE_FLAG_OVERLAPPED, lpOverlapped is ignored.

If hDevice is opened with the FILE_FLAG_OVERLAPPED flag,
the operation is performed as an overlapped (asynchronous) operation, and
lpOverlapped must point to a valid
OVERLAPPED structure that contains a handle to an
event object. Otherwise, the function fails in unpredictable ways.

For overlapped operations,
DeviceIoControl
returns immediately, and the event object is signaled when the operation has been completed. Otherwise, the
function does not return until the operation has been completed or an error occurs.

Return value

If the operation completes successfully,
DeviceIoControl returns a nonzero value.

Remarks

The FSCTL_DISMOUNT_VOLUME control code will attempt to dismount a volume regardless of whether or not any other processes are using the volume, which can have unpredictable results for those processes if they do not hold a lock on the volume. For information about locking a volume, see
FSCTL_LOCK_VOLUME.

The hDevice handle passed to
DeviceIoControl must be a handle to a volume, opened
for direct access. To retrieve a volume handle, call
CreateFile with the
lpFileName parameter set to a string of the following form:

\\.\X:

where X is a hard-drive partition letter, floppy disk drive, or CD-ROM drive. The
application must also specify the FILE_SHARE_READ and
FILE_SHARE_WRITE flags in the dwShareMode parameter of
CreateFile.

If the specified volume is a system volume or contains a page file, the operation fails.

If the specified volume is locked by another process, the operation fails. To prevent another process from
locking the volume, lock it as soon as you open it.

A dismounted volume has the following properties:

There are no open files.

The operating system does detect the volume.

The operating system tries to mount an unmounted volume as soon as an attempt is made to access it. For
example, a call to
GetLogicalDrives triggers the
operating system to mount unmounted volumes.

Dismounting a volume is useful when a volume needs to disappear for a while. For example, an application that
changes a volume file system from the FAT file system to the NTFS file system might use the following
procedure.

To change a volume file system

Open a volume.

Lock the volume.

Format the volume.

Dismount the volume.

Unlock the volume.

Close the volume handle.

A dismounting operation removes the volume from the FAT file system awareness. When the operating system
mounts the volume, it appears as an NTFS file system volume.

In Windows 8 and Windows Server 2012, this code is supported by the following technologies.

Technology

Supported

Server Message Block (SMB) 3.0 protocol

No

SMB 3.0 Transparent Failover (TFO)

No

SMB 3.0 with Scale-out File Shares (SO)

No

Cluster Shared Volume File System (CsvFS)

See comment

On CsvFs the node where dismount is issued will see a normal dismount sequence. On all other nodes FS will invalidate all the opened files.